The retinoblastoma tumor suppressor protein (pRB) and the related proteins p107 and p130 (collectively termed "pocket" proteins) play an established role in suppressing cell growth through inhibition of the E2F transcription factor. A role for the pRB family in cell cycle exit and muscle differentiation has also been documented. While cellular quiescence and p16INK4a-induced growth arrest appear to require combinations of "pocket" proteins, specific pRB family members have been implicated in terminal differentiation of muscle cells. However, very few direct, physiological targets have been linked to cellular quiescence, and fewer direct targets associated with differentiation have been identified. Furthermore, pRB binding to promoters has not been widely observed in cultured fibroblasts during the cell cycle, raising interesting and important questions regarding the role of pRB in tumor suppression and suggesting that pRB's tumor suppressive function may involve a much more extensive role in promoting differentiation than previously imagined. One goal of this proposal is to identify and characterize (1) direct, physiological targets of the pRB family involved in achieving cellular quiescence and p161NK4a -mediated growth arrest and (2) those gene targets that cooperate to confer irreversible cell cycle exit and terminal differentiation of muscle. It will also attempt to distinguish between those controls involved in cell cycle withdrawal and phenotypic differentiation. This will be accomplished through large-scale analyses of "pocket" protein binding to the genome of living cells (factor location analysis) during the process of cell cycle exit and differentiation, through simultaneous analysis of gene expression profiles, and through biochemical dissection of target promoters. By examining three cell cycle exit pathways that appear to require certain pRB family members but not others, this work will have a fundamental impact on our understanding of the existence of gene regulatory networks effecting cell cycle exit in response to distinct biological cues. pRB plays a well-documented role in growth control, and inactivation of this tumor suppressor has been associated with a large proportion of human cancers. This Proposal is therefore highly relevant to our understanding of tumor suppressive mechanisms and cancer.